Chronometers. 331 
with the view to introduce glass into the manufacture of chronome- 
ters, as a substitute for steel and brass in the balance and balance- 
springs of those machines. After a trial of more than three years, 
conducted at the Royal Observatory by the direction of the Lords 
Commissioners of the Admiralty, with two chronometers of this new 
construction, we have requested that they may be returned to us for 
further improvement. Thus the first public trial of the glass spring 
and its balance may be considered as closed ; and with your permis- 
sion, we will here briefly advert to some of the progressive experi- 
ments with it, which have been detailed in former numbers of your 
work, beginning with a short statement of the first application of a 
balance-spring to a watch. 
About the year 1660, Hooke first applied a spring to the balance 
of a watch, so that the vibrations were returned. It also admitted of 
the watch being regulated, and it remains the same at the present 
time in watches, as when he first applied it. Its form is that of a 
flat spiral, and made of steel. The effect of heat on this spring’ will 
be seen in one of the following tables, which for 68° of Fahrenheit, 
amounts to 385 seconds in 24 hours. ‘To reduce this quantity, our 
present experiment with glass zs the first that has been made. 
While at the temperature of 68°, the error from a steel balance 
and spring is 385 seconds in 24 hours; that of glass is only 40 sec. 
It is also clearly desirable to introduce a balance-spring the elasticity 
of which is not produced by any chemical or mechanical process, and 
which is free from magnetic influence, and capable of resisting the 
effects of corrosion. By its adoption, the principle of the chronometer 
is altogether changed ; and it is remarkable, that while the improve- 
ments of the mechanism in a chronometer have been many and va- 
rious, the present is the on/y attempt to reduce the errors of the bal- 
ance-spring, or, in other words, to reconstruct the chronometrical 
"part of the machine de novo, and however chimerical the introduc- 
_ tion of glass appeared in the first instance, it will be seen that we 
have made considerable advancement in the three years’ trial. In 
that period it has undergone the most severe test from the discharge 
of guns on board ship, it has also been continually going for upwards 
of three years without fracture, and, we may add, has been of great 
service to Captain Hewett in his survey of the North Sea. This offi- 
cer states, in a private letter accompanying his official report on it, 
“that the daily rates derived from the comparison with his standard 
chronometer, perhaps never were excelled in chronometrical jour- 
nal. ” 
